Jet Quenching and QGP

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Jet Tomography

Ordinary nuclear matter is predicted to go through a phase transition to a completely new state under extremely high temperature and density. Such a state of deconfined quarks and gluons soup should have existed about few micro-second after the Big Bang. Nuclear physicists are trying to recreate such a state of matter in the laboratory by colliding two heavy nuclei at extremely high energy at the Relativistic Heavy-ion Collider or nicknamed RHIC at Brookhaven National Laboratory.

Once created in the laboratory, one has to find a way to verify and study it. There are many proposed signals of QGP that one can look for. One of them is called jet quenching. Its concept was initially developed at Lawrence Berkeley National Laboratory by Miklos Gyulassy (now at Columbia University), Micheal Pluemer (now in Germany) and myself [ 1, 2] in 1989-1991. It is very similar to the idea of computed tomography (CT) used in medical industry.

Together with the bulk matter formed in heavy-ion collisions, a pair of very energetic quarks or gluons are also produced. These pair of particles or jets will propagate through the dense medium and sometimes get slowed down or absorbed, much like X-ray goes through a tissue sample. By detecting and studying the attenuation of these jets or jet quenching, one should be able to know the properties of the produced dense medium.

Modified Jet Fragmentation

Miklos Gyulassy and I carried out the first pQCD calculation of energy loss due to induced radiation [3]. Since then many theoretical studies have been made and many theoretical problems were solved along the way.

One significant difference between jet quenching and X-ray CT is that one cannot directly measure the softening of the jet energy after it emerges from the medium. This is because of property of strong interaction called confinement that does allow existence of free quarks and gluons. The initial quark or gluon will simply become a jet of collimated hadrons. The total energy will remain because of the law of energy conservation. The only measurable thing is the change of hadron distribution within the jet. This is what we called modified fragmentation due to jet quenching and was proposed by Ina Sarcevic, Zheng Huan and myself around 1996 [4]. Detailed calculation of the modified fragmentation based on perturbative QCD was carried by Xiaofeng Guo and myself in 2000 [5].

Importance of d+Au Collisions

Just as one has to calibrate the initial flux of X-ray in CT, one should also determine the luminosity of the initial parton beams that produced the initial jets. Miklos Gyulassy and I first pointed out this importance in 1990 [2] and later I also first point out the importance of multiple scattering 1998 [6], both can be addressed by d+Au collisions at RHIC. Experiments on d+Au essentially provide a baseline for extraction of jet energy loss from Au+Au collisions.

Current Experimental Status

Jet quenching has been discovered by RHIC experiments in the first year of RHIC operation. Not only the inclusive hadron spectra are suppressed, but also the modified fragmentation functions are measured. New d+Au data from PHENIX, PHOBOS and STAR all confirmed that the observed jet quenching is due to jet interaction with the produced dense matter in heavy-ion collisions. My jet tomography analysis of the data [7] indicate that the jet energy loss is about 30 times higher than what is measured in a cold nuclei. This means that the initial particle density is about 30 times higher than a cold nucleus.

Combined together with the observed elliptic collective flow, one can easily conclude that a high density and strongly interacting matter has been produced in Au+Au collisions at RHIC. Such a matter cannot be in any other state but a quark gluon plasma within our current standard theory of strong interaction.

Future Works

This is only the beginning of using jet tomography to study properties of the dense matter in heavy-ion collisions. Much more sophisticated techniques have been developed (for example azimuthal anisotropy as I first proposed in 2000 [8]) now for detailed tomography study using multiple particle correlations, direct measurement of parton energy loss and excitational behavior of the jet quenching. Together with other measurements like electro-magnetic signals and J/Psi suppression, one will be able to map out detailed properties of the matter, including the Equation of State.

References

(1) JET QUENCHING IN DENSE MATTER.
By Miklos Gyulassy, Michael Plumer (LBL, Berkeley). LBL-28531, Feb 1990. 11pp.
Published in Phys.Lett.B243:432-438,1990

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(2) GLUON SHADOWING AND JET QUENCHING IN A + A COLLISIONS AT S**(1/2) = 200-GEV.
By Xin-Nian Wang (Duke U.), Miklos Gyulassy (LBL, Berkeley). DUKE-TH-91-25, LBL-31619, (Received Jan 1991). 9pp.
Published in Phys.Rev.Lett.68:1480-1483,1992

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(3) MULTIPLE COLLISIONS AND INDUCED GLUON BREMSSTRAHLUNG IN QCD.
By Miklos Gyulassy (Columbia U.), Xin-nian Wang (LBL, Berkeley). CU-TP-598, LBL-32682, Jun 1993. 47pp.
Published in Nucl.Phys.B420:583-614,1994
e-Print Archive: nucl-th/9306003

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: Abstract and Postscript and PDF from arXiv.org (mirrors: au br cn de es fr il in it jp kr ru tw uk za aps lanl )
: Nuclear Physics Electronic

(4) JET QUENCHING IN THE OPPOSITE DIRECTION OF A TAGGED PHOTON IN HIGH-ENERGY HEAVY ION COLLISIONS.
By Xin-Nian Wang (LBL, Berkeley), Zheng Huang, Ina Sarcevic (Arizona U.). LBL-38455, AZPH-TH-96-09, Feb 1996. 4pp.
Published in Phys.Rev.Lett.77:231-234,1996
e-Print Archive: hep-ph/9605213

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: Abstract and Postscript and PDF from arXiv.org (mirrors: au br cn de es fr il in it jp kr ru tw uk za aps lanl )
: CERN Library Record
: Phys. Rev. Lett. Server

(5) MULTIPLE SCATTERING, PARTON ENERGY LOSS AND MODIFIED FRAGMENTATION FUNCTIONS IN DEEPLY INELASTIC E A SCATTERING.
By Xiao-feng Guo (Kentucky U.), Xin-Nian Wang (LBL, Berkeley). LBNL-45631, LBL-45631, May 2000. 4pp.
Published in Phys.Rev.Lett.85:3591-3594,2000
e-Print Archive: hep-ph/0005044

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: Abstract and Postscript and PDF from arXiv.org (mirrors: au br cn de es fr il in it jp kr ru tw uk za aps lanl )
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(6) SYSTEMATIC STUDY OF HIGH P(T) HADRON SPECTRA IN P P, P A AND A A COLLISIONS FROM SPS TO RHIC ENERGIES.
By Xin-Nian Wang (LBL, Berkeley). LBNL-42545, LBL-42545, Nov 1998. 27pp.
Published in Phys.Rev.C61:064910,2000
e-Print Archive: nucl-th/9812021

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(7) HIGH P(T) HADRON SPECTRA, AZIMUTHAL ANISOTROPY AND BACK TO BACK CORRELATIONS IN HIGH-ENERGY HEAVY ION COLLISIONS.
By Xin-Nian Wang (LBL, Berkeley). LBNL-52533, May 2003. 4pp.
e-Print Archive: nucl-th/0305010

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: Abstract and Postscript and PDF from arXiv.org (mirrors: au br cn de es fr il in it jp kr ru tw uk za aps lanl )

(8) JET QUENCHING AND AZIMUTHAL ANISOTROPY OF LARGE P(T) SPECTRA IN NONCENTRAL HIGH-ENERGY HEAVY ION COLLISIONS.
By Xin-Nian Wang (LBL, Berkeley). LBNL-46795, Aug 2000. 9pp.
Published in Phys.Rev.C63:054902,2001
e-Print Archive: nucl-th/0009019

: References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Citation Search
: Abstract and Postscript and PDF from arXiv.org (mirrors: au br cn de es fr il in it jp kr ru tw uk za aps lanl )
: CERN Library Record
: Phys. Rev. C Server